在Angularjs $ watch中使用Closure

Question

我正在嘗試在$ watch內部使用閉包（它用於監視下拉列表中發生的更改）。 我正在嘗試為第一次運行的變量設置一個值，然后將其替換為下拉列表中發生的更改。 我覺得$ watch中的閉包不能正常工作。 請在這里看看：

    $scope.$watch('myDropDown', function () { // it's watching the dropdown
        $scope.monthsGroup = $scope.yearAndMonthsGroup.months; // months loaded in the dropdown

        $scope.month = $scope.monthsGroup[0];// set the first month as default
        var runOnlyOnce = (function () {
            var called = false;
            return function () {
                if (!called) {
                    $scope.month = $scope.monthsGroup[$scope.monthsGroup.length -1]; // set the last month as default for first running
                    console.log("Run it!");//it is running it over again when changes occur in the dropdown
                    called = true;
                }
            }
        })();
    });`

http://jsfiddle.net/svaos5d9/

謝謝！

Answer 1

對於創建的每個實例， runOnlyOnce實際上僅運行一次。 問題在於您正在創建許多實例，每次觸發手表時都會創建一個實例。

只需將runOnlyOnce創建代碼放在表外 ，然后在內部調用即可 ：

var runOnlyOnce = (function(){
    var called = false;
    return function(){
        if(!called){
            $scope.month = $scope.monthsGroup[$scope.monthsGroup.length -1]; // set the last month as default for first running
            console.log("Run it!");//it is running it over again when changes occur in the dropdown                        
            called = true;
        }
    }
})();

$scope.$watch('myDropDown', function () {
    $scope.monthsGroup = $scope.yearAndMonthsGroup.months; // months loaded in the dropdown
    $scope.month = $scope.monthsGroup[0];// set the first month as default
    runOnlyOnce();
});